1. Field of the Invention
The present invention relates to Joule-Thomson cryostats. More specifically, the present invention relates systems and techniques for improving the response time of Joule-Thomson cryostats.
While the present invention is described herein with reference to illustrative embodiments for particular applications, it should be understood that the invention is not limited thereto. Those having ordinary skill in the art and access to the teachings provided herein will recognize additional modifications, applications, and embodiments within the scope thereof and additional fields in which the present invention would be of significant utility.
2. Description of the Related Art
A cryostat is an apparatus which provides a localized low-temperature environment in which operations or measurements may be carried out under controlled temperature conditions. Cryostats are used to provide cooling of infrared detectors in guided missiles, for example, where detectors and associated electronic components are often crowded into a small containment package. Cryostats are also used in superconductor systems where controlled very low temperatures are required for superconductive activity.
A Joule-Thomson cryostat is a cooling device that uses a valve (known in the art as a "Joule-Thomson valve") through which a high pressure gas is allowed to expand via an irreversible throttling process in which enthalpy is conserved, resulting in lowering of its temperature.
The simplest form of a conventional Joule-Thomson cryostat typically has a fixed-size orifice in the heat exchanger at the cold end of the cryostat such that cooling by the cryostat was unregulated. The input pressure and internal gas flow dynamics established the flow parameters of the coolant through the cryostat. Although the conventional Joule-Thomson cryostat is a simple apparatus in that it has no moving parts, the inherent, uncontrolled flow characteristics make the fixed-orifice type cryostat unsuitable for many applications where rapid cool-down and long cooling durations from a limited size gas supply source are required. Rapid cool-down requires high rate gas flow and a large size orifice, while long cooling durations require low gas flow rates and a small size orifice. These two conditions cannot be simultaneously met in a fixed orifice cryostat.
Since approximately the 1950's, demand-flow Joule-Thomson cryostats with internal, passive, thermostatic control of variable orifice size have been used. These cryostats have gas throttling valves which provide the ability to start cool-down with the maximum orifice size, thereby providing high rate gas flow and refrigeration for rapid cool-down. After cool-down is achieved, the orifice size is reduced by the valve for minimal gas flow rate and sustained cooling for the thermal load.
The gas throttling valve includes a thermostatic element within the mandrel of the apparatus which provides self-regulation of gas flow based upon the temperature in and around the gas plenum chamber. The cooling rate is proportional to the mass flow rate of gas through the cryostat. The thermostatic element, is conventionally a gas-filled bellows or a segment of material which contracts or expands based upon temperature. The thermostatic element is coupled to a demand-flow needle valve mechanism. As the temperature drops, the element is adapted to contract and cause the needle to extend into and partially close the Joule-Thomson orifice. At the predetermined critical temperature, the thermostatic element closes the needle valve entirely. As the temperature rises, the element expands again and actuates the valve mechanism, allowing new coolant flow through the orifice and ultimately to the heat load.
Rapid closure of the valve is needed once cooldown has been achieved with liquid cryogen collected in the cold well. If rapid closure is not achieved, gas continues to flow at a very high rate, producing a high back pressure over the liquid cryogen within the cold well due to the flow restriction past the heat exchanger of the cryostat. The high pressure on the surface of the liquid cryogen raises its boiling temperature and delays cooldown to the final equilibrium temperature required by the device being cooled. If the device being cooled is an infrared detector, the operating temperature must be cold and stable to demanding specifications. Cooldown is not achieved until the detector is cooled to its final stable operating temperature. Thus, it is important to rapidly reduce the liquid cryogen boiling pressure and temperature to improve cooldown time.
Unfortunately, conventional thermostatic elements are generally slow requiring as much as 5-10 seconds for closure after gas flow is initiated. Increases in speed of closure come at the expense of travel, the distance the element moves over its operating range to effect closure. Generally, greater travel is preferred to effect abrupt closure of the needle valve notwithstanding high flow rates of the working fluid.
Thus, a need exists in the art for a responsive thermostatic element for a Joule-Thomson cryostat which affords high speed of closure, abrupt stoppage and significant travel at low weight and cost.